contrib/difftool: use a separate config namespace for difftool commands

contrib/difftool: use a separate config namespace for difftool commands

Some users have different mergetool and difftool settings, so teach
difftool to read config vars from the difftool.* namespace.  This allows
having distinct configurations for the diff and merge scenarios.

We don't want to force existing users to set new values for no reason
so difftool falls back to existing mergetool config variables when the
difftool equivalents are not defined.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>